This study demonstrates that the mechanisms of microbiologically influenced corrosion (MIC) by Desulfovibrio vulgaris, a sulfate reducing bacterium (SRB), against X65 carbon steel and pure copper belong to two different types of MIC. Type I MIC involves extracellular electron transfer across cell walls of sessile cells in biofilms. This type of MIC is also called extracellular electron transfer MIC (EET-MIC). Type II MIC, also known as metabolite MIC (M-MIC), is caused by secreted corrosive metabolites that are more concentrated locally under a biofilm. The corrosive metabolites secreted by planktonic cells can also contribute to Type II MIC. The metabolites oxidize metals extracellularly without biocatalysis or EET. Experimental data in this work show that 20 ppm (w/w) riboflavin, a universal electron mediator, did not enhance sessile cell growth, but it accelerated EET-MIC by D. vulgaris in the ATCC † 1249 medium against X65 carbon steel with a 90% increase in weight loss and a 284% increase in the average maximum pit depth. However, 20 ppm riboflavin did not increase copper MIC, because copper MIC by SRB was due to secreted metabolites (i.e., M-MIC) rather than the direct result of sulfate reduction. This work also shows that copper MIC weight loss caused by the SRB was at least one order of magnitude higher than that of X65 carbon steel even though the SRB sessile cell count on copper was 10 times lower.
The awareness of MIC increased significantly in the past decade in many fields such as the oil and gas industry, water utilities, and the biomedical implant industry.1-3 Sulfate is a ubiquitous oxidant (electron acceptor) in seawater, brackish water and even in human saliva. Thus, SRB are often found in anaerobic environments because they can utilize this readily available oxidant.4,5 Most environments in the oil and gas industry such as reservoirs and oil transport pipelines are strictly anaerobic.6 In systems that are open to the air, SRB may grow underneath aerobic biofilms such as an iron oxidizing bacterium biofilm.7,8 Although SRB are often blamed for MIC in various settings, nitrate reducing bacteria and acid producing bacteria are shown to be corrosive as well.9-11